When you look at my portfolio page, I hope you agree that the different length of excerpts doesn’t look great. Sometimes I can’t write whatever, and is empty. How can I adjust exceprts in Brightbox to be the same? By the number of characters / words or lines?

the number 30 should resemble the number of words to be put in your excerpt. Be sure you don’t have a custom excerpt set (this can be done when creating a post or page => at the bottom of the editor), because the code I’m giving you gets overwritten by the custom excerpt.